This invention relates to gas turbine engines and particularly to the logic of an electronic fuel control that monitors certain engine operating parameters indicative of torque and closes the loop of fuel flow to provide a given torque schedule for engine start-up.
As is well known in the jet engine control art, in engine start-up, that is, from the point that the engine is ignited to point where the engine reaches idle speed, the fuel control establishes a predetermined schedule. Heretofore, such fuel control schedules were open looped where a given power lever position initiated a built-in schedule that was designed to predict the engine's operating performances. Needless to say, such a built-in schedule was not absolutely accurate as it didn't account for variations in performance of different engines of a given engine model or control deterioration and the like.
Thus, in the typical fuel control, say, as examples, the model JFC-12, JFC-25 and JFC-60 manufactured by the Hamilton Standard Division of United Technologies Corporation, to which this patent application is assigned, or the AJ-H1, CJ-G5, CJ-G8 manufactured by the Bendix Energy Controls Division of the Bendix Corporation, the single fuel flow schedule is open looped and generally, merely biased by ambient air temperature. As is well known, the starting characteristics not only must account for the above mentioned parameters, it must also account for other functions of engine internal parameters, such as temperature and pressure. To do otherwise would be comprising engine start-up where such occurs over a wide range of starting requirements.
With the advent of electronic controls and its fast time responsiveness, the capability of monitoring certain engine operating parameters and optimizing engine start-up over a wide range of starting requirements has become a reality. It is important that the engine acceleration to idle be as fast as possible without incurring stall or surge. Thus, according to this invention, it is contemplated that engine start-up scheduling is achieved by monitoring given engine requirements and closing the loop on the monitored parameters as calculated to provide start-up scheduling by adjusting fuel flow and achieving stall-free and repeatable engine acceleration. It is contemplated within the scope of this invention that a torque error signal, indicative of the difference between a desired torque schedule obtained as a function of corrected rotor speed (N/.sqroot..theta.), Mach No., and compressor inlet pressure (CIP) or compressor discharge pressure (CDP) and the actual torque, N acceleration times a constant, minus the torque utilized for aircraft accessories, and torque supplied by a starter, be used to provide the required fuel flow to provide acceleration.
Thus, according to this invention the condition of the engine prior to and during the start cycle is continuously monitored. The referenced and sensed parameters maintain the optimum scheduling of fuel throughout the start by providing the optimum acceleration characteristics without stalling the compression system.